This invention relates to an enhanced condensation heat transfer device, a shell-tube type heat exchanger with an enhanced heat transfer surface on the tube outer side, and a method for enhanced condensation heat transfer.
Indirect transfer of heat between fluids involves three resistances. A first resistance is associated with the high temperature heat source, a second resistance is imposed by the medium which separates the fluids, and a third is associated with the low temperature heat sink. For systems which allow the use of a material with high thermal conductivity, the resistance of the separating medium to the transfer of heat is small, therefore, the rate at which heat is transformed generally is controlled by the flow conditions and properities of the fluid mediums. Relative to the low temperature heat sink, coefficients in the order of 1000 BTU/hr, ft.sub.2, .degree.F. are achievable in sensible heat transfer. For processes involving a boiling low temperature medium, which practice the technology of Milton U.S. Pat. No. 3,384,154 or Kun et al. U.S. Pat. No. 3,454,081, coefficients of 8,000 to 12,000 BTU/hr, ft.sup.2, .degree.F. are achievable. The resistance associated with the high temperature heat source often controls the rate of heat transfer, particularly in processes involving condensation, wherein coefficients of less than 500 BTU/hr, ft.sup.2, .degree.F. are commonly encountered. In such systems, the liquid film which forms on the condensing surface represents the major resistance to heat transfer, and is particularly high in shell and tube equipment, wherein condensation occurs external of the tubes and drains from the surface under the influence of gravity.
The prior art teaches a variety of surface configurations which enhance heat transfer rates in processes involving condensation, wherein the condensate drains from the surface under the influence of gravity. Shell side condensation in shell and tube heat exchangers exemplifies such processes.
Gregorig ("An Analysis of Film Condensation on Wavy Surfaces" Zeitschrift fuer Angewande Mathematik and Physik, Vol. 4, pp. 40-49, teaches a method which relies on the pressure gradient associated with variations in liquid surface profile due to surface tension. Its general principles have successfully been applied to design a number of configurations which enhance the rate of condensing heat transfer. Gregorig's work was based on steam condensation and utilized a surface construction of specific dimensions, as indicated by his mathematical derivations, to obtain maximum condensation efficiency. The Gregorig surface is for application on the outer condensing surface of vertically oriented condensation tubes and its configuration can be described as a series of alternatives, rounded crests and valleys which extend axially over the length of the tube. In the vicinity of the crest region, the convexity of the heat transfer surface causes an overpressure of the condensate film's fluid pressure relative to a flat liquid surface. The higher pressure of the condensate results from its surface tension and the convex curvature of the film. In the "valley" region, a lower pressure exists due to the concave surface curvature. A resulting pressure gradient is set up in the direction of crest to valley, so that liquid condensing in the neighborhood of the crests flows readily into the valleys to flow there through under the influence of gravity. The overall effect minimizes the condensate film thickness of the crests with a corresponding increase of the heat transfer coefficient.
The surfaces which have been developed to exploit the teachings of Gregorig involve grooved, finned and channeled configurations, and require appreciable alteration of the primary heat transfer structure and present fabricational and economic drawbacks. Expectedly, the systems reflect concern regarding the ease with which the collected condensate is drained from the system, and are restricted to drainage means which constitute an unimpeded flow path for condensate egress.
A second approach to enhancing condensing heat transfer relates to means of increasing the fluid turbulence in the condensate film. In a study of a surface roughened by cutting left and right-handed threads on the outside surface of a pipe, Nicol and Medwell ("Velocity Profiles and Roughness Effects in in Annular Pipes", Journal Mech. Eng. Science, Vol. 6, No. 2, pp 110-115, 1964) discovered that the friction factor-Reynolds Number relationship resembled that of the sand-roughened pipes studied by Nikuradse ("Stromingegesetze in rauben Rohren", Forech Arb. Ing. Wes. No. 361, 1933). It is known that "mirror" image close packed sand-grain roughened surfaces enhance sensible heat transfer by disrupting the sublayer of the fluid boundary layer, thereby reducing its depth and its resistance to the transfer of heat (Dipprey, P. and Sabersky, R., "Heat and Momentum Transfer in Smooth and Rough Tubes at Various Prandtl Numbers", Int. Journal, Heat and Mass Transfer, Vol. 6, pp 329-353, 1963). Accordingly, in a condensing heat transfer study of the Nicol-Medwell roughened surface ("The Effect of Surface Roughness on Condensing Steam", Canadian Journal of Chem Eng., pp 170, 173, June, 1966), the data was analyzed on the basis of the turbulence promoting effect which sand-grained roughened surfaces are known to exert on the laminar sublayer. Nicol and Medwell measured localized heat transfer coefficients which were 400% of smooth tube performance, however, over the greater extent of the tested 8 ft long tube, values in the order of only 200% of smooth tube performance were obtained. A 200% enhancement represents a marginal improvement relative to the performance reported for Gregorig type surfaces and, therefore, the Nikol-Medwell technology has not excited commercial interest.
An object of this invention is to provide an enhanced heat transfer device having a condensation heat transfer coefficient substantially higher than obtained by the prior art.
Another object is to provide a heat transfer device characterized by high condensation coefficient, which is relatively inexpensive to manufacture on a commercial mass-production basis.
Still another object is to provide an improved shell-tube type heat exchanger characterized by enhanced condensation heat transfer means on the tube outer surface.
A further object of this invention is to provide a method for enhanced condensation heat transfer in a heat exchanger wherein a first fluid is condensed and drained from the one side of a metal wall by heat exchange with a colder second fluid on the other side of said metal wall.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.